1. Field of the Invention
The present invention relates to a video decoder for restoring video data of an encoded and multiplexed bit stream, and more particularly, to a video decoder having an interfacing function for picture synchronization which can synchronize a decoding and display operation in units of a picture to precisely control the video decoder. The present application is based on Korean Patent Application No. 96-20792, which is incorporated herein by reference.
2. Description of the Related Art
The MPEG-II standard for motion picture compression presents the standard for a high-definition television. A video decoder adopting the standard decodes and displays video data among transport stream (TS) data obtained by multiplexing video data and audio data encoded in the form of each bit stream with respect to a plurality of programs. The TS stream includes video data encoded in units the size of a picture, a picture start code (PSC) indicating a picture start, and time stamp information, in the form of a packetized elementary stream (PES) packet. The PES is defined in the system standard DIS 13818-1 in the MPEG-II standard in order to packetize and multiplex video data encoded in the bit stream form in units of a constant length or variable length packet. The time stamp information relates to the decoding time and reproducing output time, which includes decoding time stamp (DTS) and presentation time stamp (PTS) information, respectively. The PTS and DTS information is contained in the PES packet header, and may not be transferred for every picture. When a transfer is made, the PTS and DTS information is transferred with respect to an intra-coded I-picture and a predictive-coded P-picture. Only the PTS is transferred with respect to a bidirectionally predictive-coded B-picture since the PTS and the DTS are the same. The video decoder decodes the PTS and DTS information to thereby perform a control operation in units the size of a picture. The PTS represents the time for displaying the decoded picture and the DTS represents the start time for decoding a bit stream. Thus, a normal operation of the video decoder can assuredly prevent decoding and display operations from temporally leading and lagging using the DTS and PTS whenever the picture is decoded and displayed.
FIG. 1 is a block diagram showing a general video decoder. The general video decoder receives the TS stream transmitted via a channel. A TS demultiplexer 11 separates the video data and the time stamp information encoded in the bit stream form, corresponding to a program selected by a user, from the received TS stream and outputs the separated result. A bit buffer 12 stores the video data encoded in the bit stream form which has been separated by the TS demultiplexer 11. A time stamp buffer 13 stores the time stamp information which has been separated by the TS demultiplexer 11. The bit buffer 12 outputs the stored video data encoded in the bit stream form whenever a variable-length decoder 14 requests data. At the same time, the time stamp buffer 13 outputs the time stamp information corresponding to the video data output from the bit buffer 12. Accordingly, the timing of the decoding and display operations of the output video data is synchronized in units the size of a picture.
The variable-length decoder 14 decodes the time stamp information output from the time stamp buffer 13 to obtain the PTS and DTS information, outputs the PTS information to a display (not shown), and performs a variable-length-decoding operation with respect to the encoded video data output from the bit buffer 12 using the DTS information. An inverse quantizer 15 and an inverse discrete cosine transformer (IDCT) 16 inversely quantizes and inversely discrete cosine transforms the variable-length-decoded video data. The data output from the IDCT 16 is supplied to an adder 17. A frame memory 18 stores the video data output from the adder 17. A motion compensator 19 performs a motion compensation operation with respect to the video data stored in the frame memory 19 using a transmitted motion vector (not shown). The adder 17 adds the inversely quantized and inverse discrete cosine transformed data and the video data motion-compensated in the motion compensator 19. The video data output from the adder 17 is restored video data, and is both output to the display and stored in the frame memory 18. The display starts a display operation of the corresponding restored video data using the PTS information obtained from the variable-length decoder 14.
However, the above video decoder stores the bit stream video data and the time stamp information in different buffers and reads the same therefrom. Thus, when the time stamp information is used for decoding and display operation control of the bit stream video data of the corresponding picture, synchronization may be lost. Furthermore, it is difficult to effectively use the time stamp information for decoding and display operation control when errors occur, which causes difficulty in reproducing a normal picture.